A conventional printed circuit board (PCB) has lands for surface-mounted devices (SMDs), wherein the lands each comprises a predetermined pattern of copper foils, and non-electrolysis Ni--Au plates covering the copper foils.
In mounting SMDs such as resistance and capacitor chips, small outline packages (SOPs), connectors, quad flat packages (QFPs), etc. on the PCB, cream solder is supplied to the lands on the PCB for a re-flow process carried out by heating the PCB, so that the SMDs are mounted on the PWD in accordance with the bonding to the patterned SMD lands. The PCB thus mounted with the SMDs is connected to an external circuit by using a flexible printed circuit (FPC) connector.
However, the conventional PCB has disadvantages in that only packaged devices can be mounted thereon due to the application of heat for the re-flow process, high density mount of the SMDs can not be realized, as requested by user due to the usage of the FPC connector, and thickness, volume and weight are totally greater than specified ones.
In consideration of the disadvantage as described above, an outer lead bonding (OLB) method using tape carrier packages, and a chip on board (COB) method using bonding wires have been adopted in place of the re-flow method using cream solder, and the FPC connector has been replaced by a thermal pressing method providing heat-seal.
In this situation, surface-treatments for PCBs adapted to the above described methods will be explained below.
(1) Non-electrolysis Ni--Au plate
All conventional SMDs can be mounted on a PCB by using SMD lands covered with the non-electrolysis Ni--Au plates. In this type of PCBs, although no problem occurs in the thermal pressing method providing heat-seal, some problems occur in the OLB method using tape carrier packages and the COB method using bonding wires. In the OLB method using the tape carrier packages, one of the problems is the occurrence of bridges among fine and narrowly spaced outer leads caused by local re-flow of cream solder supplied on the non-electrolysis Ni--Au plates covering patterned copper foils. In case where no cream solder is used, electrodes of tape automated bonding (TAB) devices are required to be plated by non-electrolysis Ni--Au layers, so that the TAB devices are directly bonded to the PCB. However, this increases the cost of fabricating device-mounted PCBs due to the adoption of the non-electrolysis Ni--Au plates on the TAB electrodes. In the COB method using bonding wires, Au bonding wires are bonded to the non-electrolysis Ni--Au plates of the SMD lands on the PCB mainly by the bonding between Ni and Au, because Au thickness of the non-electrolysis Ni--Au plates is very thin. As a result, the mechanical strength of the Ni--Au bonding portions is very low which results in low reliability.
(2) Electrolysis solder plate
The electrolysis solder plate is of an uniform surface having no roughness thereon, so that the aforementioned bridges can be avoided, even if cream solder is supplied on mounting lands to carry out the re-flow process, although it is difficult to suppress the surplus amount of solder on the PCB. The electrolysis solder plate can be used in the OLB method using tape carrier packages without any problem. In other words, although it can be used for the SMDs and the TABs, it has a problem, when it is used for the COB method using bonding wires, because the surface uniformity thereof as required in the bonding process is not obtained. Further, the bonding strength is not high and stable among terminals due to the usage of solder, thereby resulting in deterioration in quality. As a matter of course, electrodes for applying a voltage across plating portions are required to be provided. As a result, density of circuit patterns is lowered due to the provision of the electrodes.
(3) Soft Au plate
The soft Au plate is basically provided by chemical reaction. Therefore, electrodes are required to be provided for the same reason as described in the item (2). Further, Au plate treatment is required to be made even on the side of TAB in the OLB method using tape carrier packages. As a result, the increase is inevitable in cost.
As discussed in the items (1) to (3), devices or parts to be mounted on a PCB are limited in kind dependent on what kind of a plating layer is used on the PCB. Consequently, a PCB mounted with electric elements is difficult to fabricate with high density, thin and small configuration, and high quality.